1. Field of the Invention
The present invention relates generally to the design of an aerodynamic drag reduction geometry for minimizing the drag and wake turbulence behind a tractor trailer traveling at highway speed.
It is well known from the prior art and fluid dynamics references that there is considerable drag induced at the rear of a blunt base tractor trailer traveling at moderate speed. This base drag is largely composed of a low pressure region generated from the turbulent mixing of wake vortices formed at the sides of the trailer base, and increases as the square of the forward velocity. When tractor trailers are operated as line haul (long haul) over considerable distances across the United States, the excess fuel consumed due to this base drag becomes a significant incurred cost to the transportation industry. As the cost of diesel fuel increases, any reduction in the overall drag of the United States tractor trailer fleet will produce higher efficiency and less reliance on foreign oil. Recently California Air Resources Board (CARB) has mandated use of aerodynamic devices to improve efficiency of 53-ft. refrigerated and dry van trailers by 4% and 5% fuel savings. Therefore, this mandate requires new concepts of very efficient aerodynamic devices to meet this challenge.
2. Description of the Prior Art
Over the years, there have been improvements made to reduce the aerodynamic drag of tractor trailers to decrease fuel consumption and improve efficiency of operation. These improvements have reduced the form drag of the tractor and the interference/gap drag between the tractor and trailer; however, at the rear of the trailer, where significant base drag occurs, no designs have so far gained acceptance, nor have effectively addressed the aerodynamics of turning trailer side flow into the trailer base region after 53 feet of boundary layer buildup.
Fixed shape fairings mounted at the base of the trailer have been proposed, but are cumbersome, heavy, difficult to remove and replace and have not been embraced by the trucking industry. Exhaust gas injection to the trailer base has been proposed, however, piping losses incurred to the tractor engine would negate any gains in base drag reduction. Inflatable membranes have also been proposed at the base of the trailer, but the inability to carefully shape and control the membrane surface under buffeting loads have not been addressed nor adopted as practical. Attempts have been made to make rear fairings mounted to the base of the trailer retractable, so as to permit easy loading and unloading operations. These designs, however, have consisted of complex flat panels, which are heavy, time consuming to extend or retract, and do not provide an efficient curved aerodynamic shape to transition the air flow into the base region without incurring separation and loss of effectiveness. In most of these flat panel designs, very detailed drawings and descriptions of linkages, hinges, coverings, actuators, etc., are included, but little note is given to the actual aerodynamics required in turning a turbulent flow field into a blunt base. These designs lack an understanding of the boundary layer flow present at the edge juncture of the trailer base and the control of adverse pressure gradients able to cause viscous flow separation which would render further drag reduction unlikely. Without attention to controlling the boundary layer and pressure gradients by use of a properly cambered section, curved airfoil shaped fairing, these designs prove ineffective. In general, no widespread adoption of these flat panel designs have occurred.